Flame-proof coated resistors

ABSTRACT

An electrical resistor of flame-proof type comprising a resistance material coated substrate formed with or without a plurality of grooves determinant of the value of the resultant resistor, wherein glass is applied thereon in various configurations for enabling the resultant resistor to quickly fuse to open an electrical circuit in which it is inserted.

United States Patent [191 Wada et al. Oct. 16, 1973 [54] F LAME-PROOFCOATED RESISTORS 2,672,542 3/1954 Fisher 337/167 [75] Inventors: Toshiowaaa; Yasuhiio 'Shihdo 2,302,820 11/1942 Llempt 337/185 both of Osaka,Japan [73] Assignee: Matsushita Electric industrial Co., PrimaryExami"er Harld Broome Ltd Osaka, Japan Attorney-E. F Wenderoth et al.[22] Filed: July 3, 1972 [21] Appl. No.: 268,874 [57] ABSTRACT [30]Foreign Application Priority Data An electrical resistor of flame-prooftype comprising a resistance material coated substrate formed with orJuly 7 fm without a plurality of grooves determinant of the value of theresultant resistor, wherein glass is applied [5 F' Cl 337/163 337/166337/296 thereon in various configurations for enabling the re- [5 nt.Cl. H0111 85/00 sultant resistor to quickly fuse to open an electrical[58] Field Of Search 337/163, 167, 227, circuit in which it is inserted[56] References Cited 10 Claims, 15 Drawing Figures UNITED STATESPATENTS 2,927,048 3/1960 Pritikin 338/308 PATENTEUHBT 15 1975 3.766; 508

SHEET 10F 5 F/G/ PRIOR ART FIG? FIG. 5

A it

PATENTED UCHBIQYS 3.766508 SHEET 2 BF 5 FIG 6 l I0 I E CONVENTIONAL 2 80f RESISTOR O 60 A11 5 40 O5 PRESENT RESISTOR 5 IO I5 2 0 25 310 RATE OFPOWER INCREASE 7 (TIMES) sOO- ,X

CONVENTIONAL 4OO- RESISTOR FUSING 3OO- PRESENT RESISTOR 200- *OO/ 2 5OIOO I50 200 APPLIED VOLTAGE TIME (SECONDS) FIG. 8

- CONVENTIONAL ,2 4 RESISTOR $5 PRESENT 2 RESISTOR 2 0 11 250 500 750I000 "*(HOUR) C m PAIENTEIJUBT I 6 I975 .SHEET 5 or 5 FIG. /4

CONVENTIONAL RESISTOR GLASS GLASS A PRESENT B RESISTOR "*(HOUR) RESISTOROF RESISTOR OF RATE OF POWER INCREACE (TIMES) 1 FLAME-PROOF COATEDRESISTORS The present invention relates to electrical resistors whichare flame-proof and which do not become heated to an extent liable tocause damage to other circuit elements when there is an overload in thecircuits in which they are inserted.

More specifically, the present invention is directed to an improvementin electrical film resistors coated with resistance material such asmetal, metal oxide or carbon.

Recently, flame-proof resistors have come into use, their purpose beingto avoid flaming and burning ofthe resistors when there is a severeoverload in the resistor inserted circuits, and thus to avoid damage toother circuit elements caused by such burning. One type of conventionalelectrical film resistors of the character above referred to comprises,as shown in FIG. 1 of the accompanying drawings, a substrate 1 composedof glass or a similar electrically insulating material which is usuallycylindrical in shape, a thin film 2 of electrical resistance material,for example, tin oxide, applied or coated on the entire peripheralsurface of said substrate, a cap or land 3 of metal or otherelectrically conducting material attached to each end of the coatedsubstrate 1, a lead wire 4 soldered to each one of said caps 3 andextending therefrom for the purpose of external electrical connectionand a protecting coating 5 of electrically insulating and flame-proofmaterial applied or coated on the entire surface of the coatedsubstrate. In this arrangement, the value of the resistor is determinedby the number of grooves 6 cut in the coating of resistance material onthe substrate 1, as is well known to thoseskilled in the art.

However, the flame-proof resistors employed hitherto tend to achievetheir object only partially, since, while they are flame-proof, whenthere is an over-load imposed on any one of the conventional resistorsof the above character, there is a considerable time at which they areat or'near read heat before they open the circuit with the result that,while theresistors themselves may not be subjected to damage, adjacentelements in the resistor inserted circuits may be burnt or damaged. Theproblems associated with such resistors have been mainly connected withthe coating, and research has been directed to finding new materials forsuch coating,

which, however, for the reasons stated above only partially solve theproblems, and incidentally tend to be more difficult, and moreexpensive, to produce. There are also wire-wound fuse resistors whichact as circuit openers upon receipt of overload, but in order to improvetheir fusibility their windings must be as fine as possible, with theresult that they are not very reliable under ordinary conditions, and,of course, that they are not easy to manufacture.

There is therefore a need in the electronics industry for an easily andcheaply manufactured resistor which is itself flame-proof and at thesame time is not liable to cause burning or damage to other circuitelements in the resistor inserted circuit. While there has beenconsiderable research aimed at overcoming the problems associated withthese resistors, there has been no really new approach.

It is accordingly an essential object of the present invention toprovide an improved electrical flame-proof resistor which is free of thedisadvantages inherent in the conventional resistor of the characterabove referred to.

It is another object of the present invention to provide an improvedresistor which rapidly opens the resistor inserted circuit uponapplication of a severe overload.

It is a further object of the present invention to provide an improvedresistor which does not heat unduly on severe overload prior to openingthe circuit, thus avoiding possible damage to other circuit elementsprovided on the same circuit.

It is a still further object of the present invention to provide animproved resistor which is stronger and more reliable than theconventional one.

It is a still further object of the present invention to provide animproved resistor in accordance with the objects stated above which isboth cheap and easy to manufacture.

The film resistor according to the present invention, whose constructionin other respects is conventional, provides a new approach, in that itis prevented from flaming or giving out sufficient heat to damage othercircuit elements by a coating of glass applied in various configurationson the resistor film, and moreover uses a coating material which is easyand cheap to manufacture.

More particularly, the essential features of the present invention,which relates to flame-proof resistors, such as metal coated resistors,metal oxide coated resistors or carbon coated resistors, are that glassis coated or applied in various configurations on the resistance filmcoated over the entire peripheral surface of the substrate, and thatwhen overcurrent flows the glass on the resistance film is fused by theheat evolved by the resistance film and the resistance film issubsequently melted, thus opening the resistor inserted circuit.

These and other objects and features of the present invention willbecome apparent from the following description taken in conjunction withpreferred embodiments thereof illustrated in the accompanying drawings,in which;

FIG. 1 is a longitudinal cross sectional view of the conventionalresistor employed in the description of the background of the presentinvention,

FIG. 2 to FIG. 5 are each similar view to FIG. 1 showing first to fourthembodiment of the present invention,

FIGS. 6 to 14 are each showing a characteristic comparision between theconventional resistor and the resistor of the present invention, whereinFIGS. 6, 9 and 12 are diagrams showing the relationship between thepower applied and the time-to-fusing, FIGS. 7, 10 and 13 are diagramsshowing a surface temperature characteristic of the resistors whenapplied with an electrical power of 30 wattages and FIGS. 8, 11 and 14are diagrams showing an overload standing characteristic of theresistors when applied with an electrical power of 4.5 watts, and

FIG. 15 is a diagram showing the relationship between the power appliedand the time-to-fusing of the various embodiments of the presentinvention.

It is to be noted that, for the sake of brevity, the description of thepresent invention will be hereinafter made as applied to theconventional resistor of the foregoing construction and, therefore, likereference numerals employed in FIG. 1 are concurrently employed in thedescription of the present invention to designate like parts shown.

. 3 Referring first to FIG. 2, prior to the application of theprotecting coating 5 and after the grooves 6 have been formed on, theperipheral surface of the resistance material coated substrate 1, apaste of glass, which is formed by'mixing glass with adhesive materialsolved by the use of organic solvent is applied on the resistance film 2on the peripheral surface of the substrate 1 in parallel relation to thelongitudinal axis of the resistor substrate crossing over a plurality ofgrooves 6. This paste of glass, after it has been dried, is formed intoa glass coating as designated by 7.

In the second embodiment of the present invention shown in FIG. 3, theglass coating 7, that has been applied in the similar manner as in theforegoing embodiment, is in the form of a ring surrounding asubstantially intermediate portion of the resistor substrate and has asuitable width.

lowing non-limiting examples.

EXAMPLE I Powdered glass was made into a paste. with the mixture ofadhesive material solved by the use of organic solvent and coated in themanner shown in FIG. 2 onto the metal oxide film coated on the substrateand formed with the grooves, to obtain a metal oxide coated resistor of3 watts-1K0 (external diameter 8 mm. length 23 mm.), which process wasfollowed by painting with silicon insulating paint and heat treatment.The comparision was made between the fusing characteristics, surfacetemperature characteristics and the overload standing characteristics ofthis resistor and those of conventional resistors. The results are shownin FIGS. 6 to 8. As can be seen from FIG. 6, while it was more than 10minutes before a conventional resistor fused upon application of theload of a 'value times the rated load, the resistor of the presentinvention fused in about 60 seconds upon application of the load of avalue 10 times the rated load. Further, as can be seen from FIG. 7, whenan overcurrent flows, although the resistor was at red heat, the periodin which the resistor of the present invention was at red heat wasextremely short and the resistor of the present invention was quicklyfused, whereas according to the prior art thiscondition continues for along time. FIG. 8 shows that the overload standing characteristics ofboth types of resistors are essentially the same.

EXAMPLE II Powdered glass was made into a paste with the mixture ofadhesive material solved by the use of organic solvent and coated in themanner shown in FIG. 2'on to the metal film coated on the substrate andformed with the grooves to obtain a metal coated resistor of 3 watts-I00(external diameter 8 mm. length 23 mm.), which process was followed'bypainting with silicon insulating paint and heat treatment. Thecomparision was made between the fusing characteristics, surfacetemperature characteristics and the overload standing characteristics ofthis resistor and those of conventional resistors. The results are shownin FIGS. 9 to 11. Asis shown in FIG. 9, while a conventional resistordoes not fuse even after 10 minutes beginning from the application of anelectrical power of a value not more than 10 times the rated power, theresistor of the present invention fused in about seconds. The othercharacteristics represent the similar to those of the foregoing ExampleI.

EXAMPLE in This example is illustrating the effect of the employment ofglasses with different fusion points.

Two kinds of powdered glass, which are respectively hereinafter referredto as Glass A having the temperature of fusion at 340C. and Glass Bhaving that at 480C, were mixed and made into a paste by the addition ofadhesive material solved by the use of organic solvent and applied inthe manner shown in FIG. 2 on to the metal coated resistor substrateformed with the grooves, to obtain a metal coated resistor of 3watts-100 (external diameter 8 mm. length 23 mm.), which process wasfollowed by painting with silicon insulating paint and heat treatment.The comparision was .made between the fusing characteristics, surfacetem- EXAMPLE IV Powdered glass was made into a paste with the mixture ofadhesive material solved by the use of organic solvent and then coatedon the carbon coated resistor substrate formed with the grooves toobtain a carbon coated resistor of one-eighth watt-10K!) (externaldiameter 1.7 mm., length 55 mm.), which process was followed by paintingwith silicon paint and heat treatment. The comparision was made betweenthe fusing characteristics and the overload standing characteristics ofthe resistor of the present invention and those of conventionalresistors.

The results are given below and can be seen to be generally quitesimilar to those of any one of the preceding examples.

Fusing Characteristics The time to fusing of the one-eighth watt-10K!)resistor on application of the power of a value 10 times the rated load:

Conventional Resistor-still unfused even after 5 minutes PresentResistor-fused in 10 seconds Overload Standing Characteristics Thesecharacteristics are each expressed in terms of the percentage of changein the resistance of a oneeighth watt-10K!) carbon coated resistor whenapplied with a load of one-fifth watt for a period of time given below:

259.1 5 500 hr. DOQBI- Conventional Resistor 0.64 0.96 1.20 PresentResistor 0.60 0.91 1.02

The effects on fusing characteristics of the resistor coated with theglass coating in various configurations shown in FIGS. 2 to 5 are shownin FIG. 15. The comparisions were made using 3 watts-1K0 metal oxidecoated resistors. Reference to the figure makes it clear that betterresults are obtained when the glass coating is in the configuration ofeither of FIGS. 2 and 3.

Furthermore, care must be taken that the glass herein employed haspreferably a fusing point so relatively lower that quick fusion of theresultant resistor can be ensured.

Although the present invention has been fully described in connectionwith the foregoing non-limiting examples, it is to be noted that thepresent invention should be understood as applicable to resistors whoseresistance films do not have grooves cut in them, in which case asuitable configuration for the glass applied is in the form of a ringsubstantially as shown in FIG. 3.

What is claimed is:

1. An electrical resistor comprising a substrate of electricallyinsulating material, a thin film of resistance material coated on saidsubstrate, glass applied on only a portion of said resistor film,conducting terminals provided at two ends of said resistor and leadwires soldered to said ends, respectively, and a protective coatingformed over the entire surface of said coated substrate, whereby uponreceipt of severe load said glass is fused by heat evolved by saidresistor film to destroy said resistor film thereby permitting theresistor to open circuit in which said resistor is inserted.

2. An electrical resistor as claimed in claim wherein said resistancematerial is metal oxide.

3. An electrical resistor as claimed in claim wherein said resistancematerial is metal.

4. An electrical resistor as claimed in claim 1, wherein said resistancematerial is carbon.

5. An electrical resistor as claimed in claim 1, wherein said thin filmof resistance material is provided with a plurality of groovesdeterminant of the resistance of the resultant resistor.

6. An electrical resistor comprising a substrate of electricallyinsulating material, a thin film of resistance material coated on saidsubstrate, a plurality of grooves formed in said thin film on saidsubstrate, glass applied on only a portion of said resistor film,conducting terminals provided at two ends of said resistor and lead wiresoldered to said ends, respectively, and a protective coating formedover the entire surface of said coated substrate, whereby upon receiptof severe load said glass is fused by heat evolved by said resistor filmto destroy said resistor film thereby permitting the resistor to open acircuit in which said resistor is inserted.

7. An electrical resistor as claimed in claim 6, wherein the body ofsaid glass applied on the thin film of resistance material on thesubstrate extends in parallel relation to the longitudinal axis of saidsubstrate crossing a plurality of the grooves.

8. An electrical resistor as claimed in claim 6, wherein the body ofsaid glass applied on the thin film of resistance material on thesubstrate extends in the form of a ring and located at a substantiallyintermediate portion of said substrate.

9. An electrical resistor as claimed in claim 6, wherein the body ofsaid glass applied on the thin film of resistance material on thesubstrate extends in parallel relation to and not crossing over thegrooves at a substantially intermediate portion of said substrate.

10. An electrical resistor as claimed in claim 6, wherein the body ofsaid glass applied on the thin film of resistance material on thesubstrate extends in parallel relation to the longitudinal axis of saidsubstrate and sandwiched between two adjacent grooves located at asubstantially intermediate portion of said substrate.

Notice of Adverse Decision in Interference In Interference No. 99,033,involving Patent No. 3,766, 508, T. Wada and Y. Shindo, FLAME-PROOFCOATED RESISTORS, final judgment adverse to the patentees was renderedMar. 81, 1978, as to claim 2.

[Ofiicial Gazette August 8, 1.978.]

Notice of Adverse Decision in Interference In Interference No. 99,033,involving Patent No. 3,766, 508, T. Wada, and Y. Shindo, FLAME-PROOFCOATED RESISTORS, final judgment ad.- verse to the patentees Wasrendered Mar. 81, 1978, as to claim 2.

[O ficial Gazette August 8, 1 978.]

1. An electrical resistor comprising a substrate of electricallyinsulating material, a thin film of resistance material coated on saidsubstrate, glass applied on only a portion of said resistor film,conducting terminals provided at two ends of said resistor and leadwires soldered to said ends, respectively, and a protective coatingformed over the entire surface of said coated substrate, whereby uponreceipt of severe load said glass is fused by heat evolved by saidresistor film to destroy said resistor film thereby permitting theresistor to open circuit in which said resistor is inserted.
 2. Anelectrical resistor as claimed in claim 1, wherein said resistancematerial is metal oxide.
 3. An electrical resistor as claimed in claim1, wherein said resistance material is metal.
 4. An electrical resistoras claimed in claim 1, wherein said resistance material is carbon.
 5. Anelectrical resistor as claimed in claim 1, wherein said thin film ofresistance materiAl is provided with a plurality of grooves determinantof the resistance of the resultant resistor.
 6. An electrical resistorcomprising a substrate of electrically insulating material, a thin filmof resistance material coated on said substrate, a plurality of groovesformed in said thin film on said substrate, glass applied on only aportion of said resistor film, conducting terminals provided at two endsof said resistor and lead wire soldered to said ends, respectively, anda protective coating formed over the entire surface of said coatedsubstrate, whereby upon receipt of severe load said glass is fused byheat evolved by said resistor film to destroy said resistor film therebypermitting the resistor to open a circuit in which said resistor isinserted.
 7. An electrical resistor as claimed in claim 6, wherein thebody of said glass applied on the thin film of resistance material onthe substrate extends in parallel relation to the longitudinal axis ofsaid substrate crossing a plurality of the grooves.
 8. An electricalresistor as claimed in claim 6, wherein the body of said glass appliedon the thin film of resistance material on the substrate extends in theform of a ring and located at a substantially intermediate portion ofsaid substrate.
 9. An electrical resistor as claimed in claim 6, whereinthe body of said glass applied on the thin film of resistance materialon the substrate extends in parallel relation to and not crossing overthe grooves at a substantially intermediate portion of said substrate.10. An electrical resistor as claimed in claim 6, wherein the body ofsaid glass applied on the thin film of resistance material on thesubstrate extends in parallel relation to the longitudinal axis of saidsubstrate and sandwiched between two adjacent grooves located at asubstantially intermediate portion of said substrate.